Seems like it is a good idea.
Don added json-procs.tcl to acs-tcl package last August, very handy.
http://cvs.openacs.org/browse/OpenACS/openacs-4/packages/acs-tcl/tcl/json-procs.tcl
Basically you need an API to register procs that can return a datasource you can convert to/from JSON.
In general, I find you still need to end up writing specific procedures since very few applications use simple objects, although maybe with acs-object-management you could manage different views and provide those with JSON.